Magnetic recording medium

ABSTRACT

Provided is a particulate magnetic recording medium for high-density recording having a low error rate and good running durability. The magnetic recording medium comprises a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder in this order on at least one surface of a nonmagnetic support. Said nonmagnetic support comprises a clay mineral coated with an organic material, and said ferromagnetic powder is a ferromagnetic hexagonal ferrite powder having an average plate diameter ranging from 5 to 40 nm or a ferromagnetic metal powder having an average major axis length ranging from 20 to 100 nm.

FIELD OF THE INVENTION

[0001] The present invention relates to a particulate magnetic recordingmedium achieving a high C/N ratio and low error rate.

BACKGROUND OF THE INVENTION

[0002] As personal computers, workstations, and the like have becomewidespread in recent years, a large amount of research has beenconducted into the magnetic tapes (known as back-up tapes) that areemployed as external recording media for recording computer data. In thedevelopment of magnetic tapes for such uses, particularly as computershave decreased in size and increased in information processingcapability, there has been a strong demand for increased recordingcapability to increase recording capacity and achieve size reduction. Inthe area of magnetic disks, as well, the rapid development ofinformation technology is spawning a demand for the development ofmagnetic disks of ever greater density and capacity.

[0003] In the magnetic recording media developed thus far, a magneticlayer comprising a ferromagnetic hexagonal ferrite powder in the form ofiron oxide, Co-modified iron oxide, or CrO₂ dispersed in a binder thatis coated on a nonmagnetic support has been widely employed. In these,the use of ferromagnetic metal powder and ferromagnetic hexagonalferrite powder as magnetic powders is known to afford good high-densityrecording characteristics. For example, in the case of magnetic disks,10 MB MF-2TD and 21 MB MF-2SD high-capacity disks employingferromagnetic metal powder with good high-density recordingcharacteristics are known. High-capacity disks employing ferromagnetichexagonal ferrite powder in the form of 4 MB MF-2ED and 21 MB flopticalsare also known. However, today, with sharp increases in the quantity ofdata being handled, even these magnetic disks do not afford adequaterecording capacity and there is demand for magnetic disks of evengreater capacity.

[0004] In the field of magnetic tapes, technologies of reducing thelayer thickness of magnetic tapes to permit high-density recording areadvancing. Numerous magnetic tapes having a magnetic layer thickness ofequal to or less than 2 μm have appeared. With the high densification ofmagnetic recording media, a demand for greater coating smoothness hascome, and the trend in magnetic material has been toward microparticles.However, when a magnetic layer of equal to or less than 2 μm inthickness is directly coated on a support, the surface of the magneticlayer is affected by additives in the magnetic layer such as abrasivesand carbon, aggregates of magnetic powder, and the nonmagnetic support,rendering the magnetic layer rough and prone to exhibit deterioration inelectromagnetic characteristics and dropout.

[0005] One means of solving this problem is to provide a nonmagneticlayer beneath the magnetic layer and thinly apply a highly concentratedmagnetic coating liquid by a simultaneous multilayer coating method(Japanese Unexamined Patent Publication (KOKAI) Showa Nos. 63-191315 and63-187418). The use of such simultaneous multilayer coating methodsyields good electromagnetic characteristics even in a particulatemagnetic recording medium having a thin magnetic layer.

[0006] However, the surface state of the nonmagnetic support in amagnetic recording medium having a thin magnetic layer greatly affectsnonmagnetic layers and magnetic layers positioned over them. Thus, whenemploying a support of poor surface smoothness, the problem of reducedrunning durability occurs in addition to problems such as reducedelectromagnetic characteristics and dropout. Further, in the magneticrecording media actually employed in digital VRC systems and DDS-4systems, for example, high variation in the thermal expansioncoefficient and moisture expansion coefficient during recording andreproduction causes variation in the thermal and moisture expansioncoefficients of the magnetic recording medium itself, creating problemsin the form of deterioration of electromagnetic characteristics anddurability. Thus, further improvement is necessary.

[0007] To improve the support, Japanese Unexamined Patent Publication(KOKAI) Heisei No. 6-234907 describes the specification of the size ofthe filler incorporated into the support, and Japanese Unexamined PatentPublication (KOKAI) Heisei No. 8-187774 and Japanese Unexamined PatentPublication (KOKAI) Nos. 2000-57558 and 2000-336186 describe the use ofclay minerals as the fillers incorporated into the support. However,none of the techniques described in the above-cited art is capable ofimproving electromagnetic characteristics and durability in aparticulate magnetic recording medium having a multilayer-configurationwith a thin magnetic layer and a nonmagnetic layer.

[0008] Accordingly, it is an object of the present invention to providea particulate magnetic recording medium for high-density recordinghaving a low error rate and good running durability.

[0009] The present inventors conducted extensive research into thethermal expansion coefficient and moisture expansion coefficient both inthe longitudinal and width directions, resulting in the discovery thatincorporating a clay mineral coated with an organic material into anonmagnetic support yielded a magnetic recording medium with littlevariation in thermal and moisture expansion coefficients that affordedgood electromagnetic characteristics and durability; the presentinvention was devised on that basis.

SUMMARY OF THE INVENTION

[0010] That is, the object of the present invention mentioned above isachieved by:

[0011] (1) a magnetic recording medium comprising a nonmagnetic layercomprising a nonmagnetic powder and a binder and a magnetic layercomprising a ferromagnetic powder and a binder in this order on at leastone surface of a nonmagnetic support, wherein

[0012] said nonmagnetic support comprises a clay mineral coated with anorganic material, and

[0013] said ferromagnetic powder is a ferromagnetic hexagonal ferritepowder having an average plate diameter ranging from 5 to 40 nm or aferromagnetic metal powder having an average major axis length rangingfrom 20 to 100 nm.

[0014] Preferred modes of the present invention are as follows;

[0015] (2) The magnetic recording medium according to (1), wherein saidclay mineral is a layered silicate compound.

[0016] (3) The magnetic recording medium according to (2), wherein saidlayer silicate compound is at least one member selected from the groupconsisting of smectite clay, swelling mica, and swelling vermiculite.

[0017] (4) A method of reproduction with a magnetoresistive (MR) head,wherein the reproduction is conducted on the magnetic recording mediumaccording to (1).

[0018] The magnetic recording medium of the present invention isdescribed in greater detail below.

[0019] [Nonmagnetic Support]

[0020] The magnetic recording medium of the present invention ischaracterized by comprising a clay mineral coated with an organicmaterial in a nonmagnetic support. The clay mineral coated with anorganic material suitable for use in the present invention can be acompound (referred to hereinafter as an “organic onium-treatedcompound”) such as a layered silicate compound that has been subjectedto the action of an organic onium ion.

[0021] In the magnetic recording medium, adsorbing a resin component toa filler in the nonmagnetic support is thought to reduce mobility (thetendency to migrate) and to improve the dimensional stability of thenonmagnetic support, and thus, of the magnetic recording medium as awhole. Here, the higher the dispersibility of the filler contained inthe nonmagnetic support, the more resin component that adsorbs to thefiller, that is, the more polymer there is tending not to migrate, andthe greater the improvement in dimensional stability thought to beachieved.

[0022] In the magnetic recording medium of the present invention, theorganic onium-treated compounds contained in the nonmagnetic supporthave completely different structures from the multilayeredmicrometer-level aggregate structure of untreated layered silicatecompounds. That is, when the layered silicate compounds are treated withorganic onium ions, organic onium ions having affinity for resin areincorporated between the layers. Thus, the areas between the layers ofthe layered silicate compound are widened, and they can be dispersed inthe resin as extremely fine, independent thin flakes, exhibitingextremely good dispersibility. In the present invention, theincorporation of organic onium-treated compounds having such gooddispersibility into the support as a filler yields a nonmagnetic supportwith good dimensional stability and little variation in thermalexpansion coefficient and moisture expansion coefficient duringrecording and reproduction. Since the nonmagnetic support with gooddimensional stability that is employed in the present inventionundergoes extremely little variation in thermal expansion coefficientand moisture expansion coefficient during recording and reproduction, amagnetic recording medium affording good electromagnetic characteristicsand durability can be obtained even when a thin magnetic layercomprising microparticulate magnetic material is coated.

[0023] Examples of the above-mentioned layered silicate compound aresmectite clay, swelling mica, and swelling vermiculite primarilycomprised of tetrahedral sheets of silicon oxide and octahedral sheetsof metal hydroxides.

[0024] The smectite clay, which may be a natural or synthetic compound,is denoted by the general formula: X_(0.2-0.6)Y₂₋₃Z₄O₁₀(OH)₂.nH₂O (whereX is one or more members selected from the group consisting of K, Na, ½Ca, and ½ Mg; Y is one or more members selected from the groupconsisting of Mg, Fe, Mn, Ni, Zn, Li, Al, and Cr; and Z is one or moremembers selected from the group consisting of Si and Al; with H₂Odenoting water molecules bonded to interlayer ions and n varying greatlywith interlayer ions and the relative humidity). Specific examples ofsmectite clay are: montmorillonite, beidellite, nontronite, saponite,iron saponite, hectorite, sorconite, stibnite, and bentonite, as well assubstitution products, derivatives, and mixtures thereof.

[0025] The swelling mica, which may be a natural or synthetic compound,is denoted by the general formula: X_(0.5-1.0)Y₂₋₃(Z₄O₁₀)(F,OH)₂ (whereX is one or more members selected from the group consisting of Li, Na,K, Rb, Ca, Ba and Sr; Y is one or more members selected from the groupconsisting of Mg, Fe, Ni, Mn, Al, and Li; and Z is one or more membersselected from the group consisting of Si, Ge, Al, Fe, and B). Thecompounds have the property of swelling in water, in a polar solventhaving any degree of compatibility with water, and in a mixed solvent ofwater and such a polar solvent. Examples are Li-type tenorite, Na-typetenorite, Li-type tetrasilicon mica, and Na-type tetrasilicon mica, aswell as substitution products, derivatives, and mixtures thereof.

[0026] Swelling vermiculite includes both trioctahedral and dioctahedralforms denoted by the general formula: (Mg, Fe,Al)₂₋₃(Si_(4-x)Al_(x))O₁₀(OH)₂.(M⁺, ½ M²⁺)_(x).nH₂O (where M denotes theexchangeable ion of an alkali or alkaline earth metal such as Mg or Na,X=0.6 to 0.9, and n=3.5 to 5).

[0027] The above-mentioned layered silicate compounds may be employedsingly or in combinations of two or more. A layered silicate compoundhaving a crystal structure with a high degree of purity that isregularly stacked along the C-axis is desirable, but mixed-layerminerals in which the crystal period is disorderly and various types ofcrystal structure are mixed together may also be employed.

[0028] The organic onium ions employed in the present invention have thestructures represented by ammonium ions, phosphonium ions, sulfoniumions, and onium ions derived from aromatic heterocycles. Theincorporation of onium ions introduces an organic structure of lowintermolecular force between the layers of the negatively chargedlayered silicate compound, increasing affinity between the layeredsilicate compound and the resin. Examples or organic onium ions arealkyl amine ions such as lauryl amine ions and myristyl amine ions; andammonium ions comprising both an alkyl group and a glycol group, such asdiethylmethyl(polypropyleneoxide)ammonium ions anddimethylbis(polyethyleneglycol)ammonium ions.

[0029] Examples of the compounds employed to supply organic onium ionsto the layered silicate compound in the present invention are: ammoniumion donors such as tetraethylammonium chloride,n-dodecyltrimethylammonium chloride, and dimethyldistearylammoniumchloride; phosphonium ion donors such as ethyltriphenylphosphoniumchloride, tetra-n-butylphosphonium bromide, and tetraethylphosphoniumbromide; and sulfonium ion donors such as trimethylsulfonium iodide andtriphenylsulfonium bromide.

[0030] Layered silicate compounds that have been treated with suchorganic onium ions may be manufactured by the known technique ofreacting organic onium ions with a layered clay mineral comprising anegative layer lattice and exchangeable cations (Japanese Examine PatentPublication (KOKOKU) Showa No. 61-5492 and Japanese Unexamined PatentPublication (KOKAI) Showa No. 60-42451).

[0031] In the present invention, the dispersion state of the organiconium-treated compound present in the nonmagnetic support can be givenby the following parameters.

[0032] First, when defining the average layer thickness as the numberaverage value of the layer thickness of the organic onium-treatedcompound dispersed in thin flake-like form, the upper limit of theaverage layer thickness of the organic onium-treated compound in thesupport is preferably equal to or less than 150 Å, more preferably equalto or less than 100 Å. Although the lower limit of the average layerthickness of the organic onium-treated compound is not specificallylimited, it is about 10 Å. When the average layer thickness of theorganic onium-treated compound is within the above-stated range, asurface can be formed that is smooth as well as affords suitableroughness.

[0033] Further, when defining the maximum layer thickness as the maximumvalue of the layer thickness of the organic onium-treated compounddispersed in flake-like form in the support, the upper limit of themaximum layer thickness of the organic onium-treated compound in thesupport is preferably 1,500 Å, more preferably 1,200 Å, and furtherpreferably, 1,000 Å. When the upper limit of the maximum layer thicknessis equal to or less than 1,500 Å, a surface can be formed that is smoothas well as affords suitable roughness. The lower limit of the maximumlayer thickness of the organic onium-treated compound is notspecifically limited.

[0034] The layer thickness mentioned above can be calculated from imagesphotographed by microscope or the like. For example, a support placed onan X-Y surface can be cut into thin strips in parallel with the X-Zsurface or Y-Z surface, the thin strips observed with a transmissionelectron microscope at a high magnification of about 40,000 to 100,000or more, and the layer thickness calculated. In measurement, an areacontaining 100 or more particles of an organic onium-treated compoundcan be randomly selected in the transmission electron microscope imageobtained by the above-described method, an image can be generated withan image processor or the like, and the thickness can be determined byprocessing the image with a computer or the like. Further, a ruler orthe like can be employed to conduct measurement.

[0035] Examples of nonmagnetic supports suitable for use in the presentinvention are known biaxially oriented polyethylene naphthalate,polyethylene terephthalate, polyamide, polyimide, polyamidoimide,aromatic polyamide, and polybenzoxidazole. Preferred examples arepolyethylene terephthalate, polyethylene naphthalate, and aromaticpolyamide. These nonmagnetic supports may be subjected in advance tocorona discharge, plasma treatment, adhesion-enhancing treatment, andheat treatment.

[0036] The center surface average roughness (JISB0660-1998,ISO4287-1997) on the magnetic layer coating side of the nonmagneticsupport suited to use in the present invention is 2 to 10 nm at a cutoffvalue of 0.25 mm, preferably falling within a range of 3 to 9 nm. Thetwo surfaces of the support may differ in roughness. The preferredthickness of the nonmagnetic support in the magnetic recording medium ofthe present invention ranges from 3 to 80 μm.

[0037] The method of preparing the nonmagnetic support in the presentinvention is not specifically limited other than that the clay mineralcoated with an organic material is added to the resin constituting thesupport and dispersed. However, the mechanical strength in thelongitudinal and width directions is desirably adjusted. Specifically,when forming (manufacturing) a film from the resin in which the claymineral coated with an organic material is dispersed, a method ofsuitably stretching in the longitudinal and width directions isdesirably employed. The Young's modulus of the support employed in thepresent invention is preferably from 4,400 to 15,000 MPa, morepreferably from 5,500 to 11,000 MPa, in both the longitudinal and widthdirections. The Young's modulus in the longitudinal direction may bedifferent from that in the width direction.

[0038] [The Magnetic Layer]

[0039] The magnetic recording medium of the present invention ischaracterized by employing, as a magnetic powder comprised in themagnetic layer, a ferromagnetic hexagonal ferrite powder with an averageplate diameter of 5 to 40 nm or a ferromagnetic metal powder with anaverage major axis length of 20 to 100 nm to achieve high-densityrecording.

[0040] <Ferromagnetic Hexagonal Ferrite Powder>

[0041] Ferromagnetic hexagonal ferrite powder has a hexagonalmagnetoplumbite structure, extremely high single-axis crystal magneticanisotropy, and extremely high coercive force (Hc). Thus, the magneticrecording medium in which ferromagnetic hexagonal ferrite powder isemployed has good chemical stability, resistance to corrosion, andresistance to friction. Furthermore, a reduction in the magnetic spacingaccompanying high densification becomes possible, permitting thinning ofthe film, and a high C/N ratio and resolution. The average platediameter of ferromagnetic hexagonal ferrite powder ranges from 5 to 40nm, preferably from 10 to 38 nm, and more preferably from 15 to 36 nm.Generally, when increasing the track density and reproducing with an MRhead, it is necessary to reduce noise and to reduce the average platediameter of the ferromagnetic hexagonal ferrite powder. From theperspective of reducing the magnetic spacing, as well, the average platediameter of the hexagonal ferrite is desirably made as small aspossible. However, an excessively small average plate diameter in theferromagnetic hexagonal ferrite powder causes unstable magnetization dueto thermal fluctuation. Thus, the lower limit of the average platediameter of the ferromagnetic hexagonal ferrite powder employed in themagnetic layer of the magnetic recording medium of the present inventionis set to 5 nm. At an average plate diameter of less than 5 nm, theeffects of thermal fluctuation are significant and stable magnetizationis hardly achieved. Additionally, the upper limit of the average platediameter of the ferromagnetic hexagonal ferrite powder is set to 40 nm.An average plate diameter exceeding 40 nm increases noise, reduceselectromagnetic characteristics, and is not suited to reproduction withmagnetoresistive (MR) heads.

[0042] The average plate diameter of the ferromagnetic hexagonal ferritepowder can be determined by photographing the ferromagnetic hexagonalferrite powder with a transmission electron microscope and directlyreading the plate diameter of the ferromagnetic hexagonal ferrite powderfrom the photograph, or by combining the method of reading platediameters by tracing a transmission electron microscope photograph withthe image analyzer known as the IBASSI made by Karl Zeiss Co. andobtaining an average from the values measured.

[0043] Examples of hexagonal ferrite ferromagnetic powders comprised inthe magnetic layer in the present invention are various substitutionproducts of barium ferrite, strontium ferrite, lead ferrite, and calciumferrite, and Co substitution products. Specific examples aremagnetoplumbite-type barium ferrite and strontium ferrite;magnetoplumbite-type ferrite in which the particle surfaces are coveredwith spinels; and magnetoplumbite-type barium ferrite, strontiumferrite, and the like partly comprising a spinel phase. The followingmay be incorporated in addition to the prescribed atoms: Al, Si, S, Sc,Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb,Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, Nb, and the like.Compounds to which elements such as Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn,Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, and Nb—Zn have been added may generallyalso be employed. They may comprise specific impurities depending on thestarting materials and manufacturing methods.

[0044] The particle size of the ferromagnetic hexagonal ferrite powderis, as an average plate diameter as mentioned above, 5 to 40 nm,preferably 10 to 38 nm, more preferably 15 to 36 nm. The mean platethickness is preferably 1 to 30 nm, more preferably 2 to 25 nm, furtherpreferably 3 to 20 nm. The plate ratio (plate diameter/plate thickness)is preferably 1 to 15, more preferably 1 to 7. If the plate ratio iswithin a range of 1 to 15, it is possible to achieve adequateorientation properties while maintaining a high filling property in themagnetic layer, as well as to prevent noise increase due to stackingbetween particles. In addition, the specific surface area by BET methodwithin the above-mentioned particle size is 10 to 200 m²/g, almostcorresponding to an arithmetic value from the particle plate diameterand the plate thickness.

[0045] For the ferromagnetic hexagonal ferrite particle, narrowdistributions of particle plate diameter and plate thickness arenormally preferred. Although difficult to render in number form, 500particles can be randomly measured in a TEM photograph of particles tomake a comparison. The distributions of the particle plate diameter andplate thickness are often not a normal distribution. However, whenexpressed as the standard deviation to the average size, σ/averagesize=0.1 to 2.0. The particle producing reaction system is rendered asuniform as possible and the particles produced are subjected to adistribution-enhancing treatment to achieve a sharp particle sizedistribution. For example, methods such as selectively dissolvingultrafine particles in an acid solution by dissolution are known.

[0046] The coercive force (Hc) of the hexagonal ferrite particle can be159.2 to 238.8 kA/m (2000 to 3000 Oe), preferably 175.1 to 222.9 kA/m(2200 to 2800 Oe), more preferably 183.1 to 214.9 kA/m (2300 to 2700Oe). However, if the saturation magnetization (σ s) of the head exceeds1.4 T, 159.2 kA/m or more is preferred. The coercive force (Hc) can becontrolled by particle size (plate diameter and plate thickness), thetypes and quantities of elements contained, substitution sites of theelement, the particle producing reaction conditions, and the like.

[0047] The saturation magnetization (σ s) of the hexagonal ferriteparticle is 40 to 80 A·m²/kg (40 to 80 emu/g). The higher saturationmagnetization (σ s) is preferred, however, it tends to decrease withdecreasing particle size. Known methods of improving saturationmagnetization (σ s) are combining spinel ferrite with magnetoplumbiteferrite, selection of the type and quantity of elements incorporated,and the like. It is also possible to employ W-type hexagonal ferrite.When dispersing the magnetic material, the surface of the magneticmaterial particles is processed with a substance suited to a dispersionmedium and a polymer. Both organic and inorganic compounds can beemployed as surface treatment agents. Examples of the principalcompounds are oxides and hydroxides of Si, Al, P, and the like; varioussilane coupling agents; and various titanium coupling agents. Thequantity of surface treatment agent added ranges from 0.1 to 10 weightpercent relative to the weight of the magnetic material. The pH of themagnetic material is also important to dispersion. A pH of about 4 to 12is usually optimum for the dispersion medium and polymer. From theperspective of the chemical stability and storage properties of themedium, a pH of about 6 to 11 can be selected. Moisture contained in themagnetic material also affects dispersion. There is an optimum level forthe dispersion medium and polymer, usually selected from the range of0.01 to 2.0 percent.

[0048] Methods of manufacturing the ferromagnetic hexagonal ferritepowder include: (1) a vitrified crystallization method consisting ofmixing into a desired ferrite composition barium oxide, iron oxide, anda metal oxide substituting for iron with a glass forming substance suchas boron oxide; melting the mixture; rapidly cooling the mixture toobtain an amorphous material; reheating the amorphous material; andrefining and comminuting the product to obtain a barium ferrite crystalpowder; (2) a hydrothermal reaction method consisting of neutralizing abarium ferrite composition metal salt solution with an alkali; removingthe by-product; heating the liquid phase to 100° C. or greater; andwashing, drying, and comminuting the product to obtain barium ferritecrystal powder; and (3) a coprecipitation method consisting ofneutralizing a barium ferrite composition metal salt solution with analkali; removing the by-product; drying the product and processing it atequal to or less than 1,100° C.; and comminuting the product to obtainbarium ferrite crystal powder. However, any manufacturing method can beselected in the present invention. The ferromagnetic hexagonal ferritepowder may be surface treated as necessary with Al, Si, P, an oxidethereof, or the like. The quantity employed desirably ranges from 0.1 to10 percent of the ferromagnetic powder, and when a surface treatment isconducted, a lubricant such as a fatty acid is desirably adsorbed in aquantity of equal to or less than 100 mg/m². An inorganic ion in theform of soluble Na, Ca, Fe, Ni, Sr, or the like may be contained in theferromagnetic powder. These are preferably substantially not contained,but at levels of equal to or less than 200 ppm, characteristics areseldom affected.

[0049] <The Ferromagnetic Metal Powder>

[0050] It is known that the ferromagnetic metal powder employed in themagnetic layer of the magnetic recording medium of the present inventionhas good high-density magnetic recording characteristics. The use offerromagnetic metal powder yields a magnetic recording medium with goodelectromagnetic characteristics. The average major axis length of theferromagnetic metal powder employed in the magnetic layer of themagnetic recording medium of the present invention ranges from 20 to 100nm, preferably from 30 to 90 nm, and more preferably from 40 to 80 nm.Ferromagnetic metal powder with an average major axis length of lessthan 20 nm undergo reduction in magnetic characteristics due to thermalfluctuation, and an average major axis length exceeding 100 nm increasesnoise and it becomes difficult to achieve a good C/N (S/N) ratio.

[0051] The average major axis diameter of the ferromagnetic metal powdercan be determined by photographing the ferromagnetic metal powder with atransmission electron microscope and directly reading the major andminor axis diameters of the ferromagnetic metal powder from thephotograph, or by combining the method of reading major axis diametersby tracing a transmission electron microscope photograph with the imageanalyzer known as the IBASSI made by Karl Zeiss Co. and obtaining anaverage from the values measured.

[0052] The ferromagnetic metal powder employed in the magnetic layer inthe magnetic recording medium of the present invention is notspecifically limited with the exception that it contains Fe (containingan alloy) as a main component. Preferred ferromagnetic metal powders areferromagnetic alloy powders having a main component in the form of α-Fe.In addition to prescribed atoms, the ferromagnetic alloy powder maycomprise the following atoms: Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo,Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd,P, Co, Mn, Zn, Ni, Sr, and B. The incorporation of at least one fromamong Al, Si, Ca, Y, Ba, La, Nd, Co, Ni, and B in addition to α-Fe isdesirable. In particular, the incorporation of Co, Al and Y ispreferred. More specifically, it is preferred that Co content rangesfrom 10 to 40 atomic percent, Al content ranges from 2 to 20 atomicpercent, and Y content ranges from 1 to 15 atomic percent relative toFe.

[0053] The above-mentioned ferromagnetic metal powders may be pretreatedwith dispersants, lubricants, surfactants, antistatic agents, and thelike prior to dispersion. Further, the ferromagnetic metal powder maycomprise a small quantity of water, hydroxides or oxides. The moisturecontent of the ferromagnetic metal powder desirably ranges from 0.01 to2 percent; the moisture content of the ferromagnetic metal powder isdesirably optimized by means of the type of binder. The pH of theferromagnetic metal powder is preferably optimized based on thecombination of binders employed. The range is normally 6 to 12,preferably 7 to 11. Inorganic ions of soluble Na, Ca, Fe, Ni, Sr, NH₄,SO₄, Cl, NO₂, NO₃ and the like are sometimes incorporated into theferromagnetic powder. These are preferably substantially not contained,but characteristics are not affected when the total quantity of each ionis about equal to or less than 300 ppm. Further, there are desirably fewpores in the ferromagnetic metal powder employed in the presentinvention; the level thereof is equal to or less than 20 volume percent,preferably equal to or less than 5 volume percent.

[0054] The crystallite size of the ferromagnetic metal powder desirablyranges from 8 to 20 nm, preferably from 10 to 18 nm, and more preferablyfrom 12 to 16 nm. The crystallite size is the average value obtained bythe Scherrer method from the half width of the diffraction peak underconditions of a CuK α1 radiation source, a tube voltage of 50 kV, and atube current of 300 mA using an X-ray diffraction device (RINT 2000series made by Rigaku Corporation). The specific surface area (S_(BET))of the ferromagnetic metal powder by the BET method is desirably equalto or greater than 30 m²/g and less than 50 m²/g, preferably from 38 to48 m²/g. Within this range, it is possible to simultaneously achieveboth good surface properties and low noise. The pH of the ferromagneticmetal powder is desirably optimized in combination with the binderemployed. The range is from 4 to 12, with from 7 to 10 being preferred.When necessary, the ferromagnetic metal powder may be surface treatedwith Al, Si, P, an oxide thereof, or the like. The quantity employed isfrom 0.1 to 10 percent of the ferromagnetic metal powder, it beingdesirable for adsorption of lubricants such as fatty acids in theapplication of a surface treatment to be equal to or less than 100mg/m².

[0055] The shape of the ferromagnetic metal powder may be acicular,granular, rice particle-shaped, or plate-shaped so long as theabove-stated characteristics about particle size are satisfied. The useof acicular ferromagnetic powder is particularly preferred. In the caseof acicular ferromagnetic metal powder, the acicular ratio is preferably4 to 12, more preferably 5 to 12. The coercive force (Hc) of theferromagnetic metal powder preferably ranges from 159.2 to 238.8 kA/m(2,000 to 3,000 Oe), more preferably 167.2 to 230.8 kA/m (2,100 to 2,900Oe). The saturation magnetic flux density preferably ranges from 150 to300 T·m (1,500 to 3,000 G), more preferably 160 to 290 T·m (1,600 to2,900 G). The saturation magnetization (σ s) preferably ranges from 140to 170 A·m²/kg (140 to 170 emu/g), more preferably 145 to 160 A·m²/kg(145 to 160 emu/g).

[0056] A ferromagnetic metal powder with a low switching fielddistribution (SFD) is desirable, with equal to or less than 0.8 beingpreferred. A SFD of equal to or less than 0.8 affords goodelectromagnetic characteristics, high output, sharp magnetic reversal,and little peak shift, which is suited to high-density digital magneticrecording. Methods of achieving a low Hc distribution include improvingthe particle size distribution of goethite in the ferromagnetic metalpowder, employing monodisperse αFe₂O₃, and preventing sintering ofparticles, and the like.

[0057] The ferromagnetic metal powder that is employed may be obtainedby known manufacturing methods, examples of which are: reducing ironoxide or water-containing iron oxide that has been treated to preventsintering with a reducing gas such as hydrogen to obtain Fe or Fe—Coparticles; reducing a compound organic acid salt (chiefly a salt ofoxalic acid) with a reducing gas such as hydrogen; thermally decomposinga metal carbonyl compound; reduction by adding a reducing agent such assodium boron hydride, hypophosphite, or hydrazine to the aqueoussolution of a ferromagnetic metal; and evaporating a metal in an inertgas at low pressure to obtain micropowder. The ferromagnetic metalpowder thus obtained is desirably subjected to a known slow oxidationtreatment. The method of reducing iron oxide or water-containing ironoxide with a reducing gas such as hydrogen and controlling the time,temperature, and partial pressure of oxygen-containing gas and inert gasto form an oxide film on the surface is preferred due to lowdemagnetization.

[0058] [Nonmagnetic Layer]

[0059] The magnetic recording medium of the present invention comprisesa nonmagnetic layer comprising a binder and a nonmagnetic powder beneaththe above-mentioned magnetic layer on the nonmagnetic support. Bothorganic and inorganic substances may be employed as the nonmagneticpowder in the nonmagnetic layer. Carbon black may also be employed.Examples of inorganic substances are metals, metal oxides, metalcarbonates, metal sulfates, metal nitrides, metal carbides, and metalsulfides.

[0060] Specifically, titanium oxides such as titanium dioxide, ceriumoxide, tin oxide, tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂O₃, α-alumina withan α-conversion rate of 90 to 100 percent, β-alumina, γ-alumina, α-ironoxide, goethite, corundum, silicon nitride, titanium carbide, magnesiumoxide, boron nitride, molybdenum disulfide, copper oxide, MgCO₃, CaCO₃,BaCO₃, SrCO₃, BaSO₄, silicon carbide, and titanium carbide may beemployed singly or in combinations of two or more. α-iron oxide andtitanium oxide are preferred.

[0061] The nonmagnetic powder may be acicular, spherical, polyhedral, orplate-shaped. The crystallite size of the nonmagnetic powder desirablyranges from 4 nm to 1 μm, preferably from 40 to 100 nm. A crystallitesize falling within a range of 4 nm to 1 μm is desirable in that itfacilitates dispersion and imparts a suitable surface roughness. Theaverage particle diameter of the nonmagnetic powder desirably rangesfrom 5 nm to 2 μm. As needed, nonmagnetic powders of differing averageparticle diameter may be combined; the same effect may be achieved bybroadening the average particle distribution of a single nonmagneticpowder. The preferred average particle diameter of the nonmagneticpowder ranges from 10 to 200 nm. Within a range of 5 nm to 2 μm,dispersion is good and good surface roughness is achieved.

[0062] The specific surface area of the nonmagnetic powder desirablyranges from 1 to 100 m²/g, preferably from 5 to 70 m²/g, and morepreferably from 10 to 65 m²/g. Within the specific surface area rangingfrom 1 to 100 m²/g, suitable surface roughness is achieved anddispersion is possible with the desired quantity of binder. Oilabsorption capacity using dibutyl phthalate (DBP) desirably ranges from5 to 100 mL/100 g, preferably from 10 to 80 mL/100 g, and morepreferably from 20 to 60 mL/100 g. The specific gravity desirably rangesfrom 1 to 12, preferably from 3 to 6. The tap density desirably rangesfrom 0.05 to 2 g/mL, preferably from 0.2 to 1.5 g/mL. A tap densityfalling within a range of 0.05 to 2 g/mL reduces the amount ofscattering particles, thereby facilitating handling, and tends toprevent solidification to the device. The pH of the nonmagnetic powderdesirably ranges from 2 to 11, preferably from 6 to 9. When the pH fallswithin a range of 2 to 11, the coefficient of friction does not becomehigh at high temperature or high humidity or due to the freeing of fattyacids. The moisture content of the nonmagnetic powder desirably rangesfrom 0.1 to 5 weight percent, preferably from 0.2 to 3 weight percent,and more preferably from 0.3 to 1.5 weight percent. A moisture contentfalling within a range of 0.1 to 5 weight percent is desirable becauseit produces good dispersion and yields a stable coating viscosityfollowing dispersion. An ignition loss of equal to or less than 20weight percent is desirable and nonmagnetic powders with low ignitionlosses are desirable.

[0063] If the nonmagnetic powder is an inorganic powder, the Mohs'hardness is preferably 4 to 10. Durability can be ensured if the Mohs'hardness ranges from 4 to 10. The stearic acid (SA) adsorption capacityof the nonmagnetic powder preferably ranges from 1 to 20 μmol/m², morepreferably from 2 to 15 μmol/m². The heat of wetting in 25° C. water ofthe nonmagnetic powder is preferably within a range of 2.0×10⁻⁵ to6.0×10⁻⁵ J/cm² (200 to 600 erg/cm²). A solvent with a heat of wettingwithin this range may also be employed. The quantity of water moleculeson the surface at 100 to 400° C. suitably ranges from 1 to 10 pieces per100 Angstroms. The pH of the isoelectric point in water preferablyranges from 3 to 9. The surface of these nonmagnetic powders ispreferably treated with Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂O₃, and ZnO.The surface-treating agents of preference with regard to dispersibilityare Al₂O₃, SiO₂, TiO₂, and ZrO₂, and Al₂O₃, SiO₂ and ZrO₂ are furtherpreferable. They may be employed singly or in combination. Depending onthe objective, a surface-treatment coating layer with a coprecipitatedmaterial may also be employed, the coating structure which comprises afirst alumina coating and a second silica coating thereover or thereverse structure thereof may also be adopted. Depending on theobjective, the surface-treatment coating layer may be a porous layer,with homogeneity and density being generally desirable.

[0064] Specific examples of nonmagnetic powders suitable for use in thenonmagnetic layer in the present invention are: Nanotite from ShowaDenko K.K.; HIT-100 and ZA-G1 from Sumitomo Chemical Co., Ltd.; DPN-250,DPN-250BX, DPN-245, DPN-270BX, DPN-550BX and DPN-550RX from Toda KogyoCorp.; titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S,TTO-55D, SN-100, MJ-7, α-iron oxide E270, E271 and E300 from IshiharaSangyo Co., Ltd.; STT-4D, STT-30D, STT-30 and STT-65C from Titan KogyoK.K.; MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, MT-100F and MT-500HDfrom Tayca Corporation; FINEX-25, BF-1, BF-10, BF-20 and ST-M from SakaiChemical Industry Co., Ltd.; DEFIC-Y and DEFIC-R from Dowa Mining Co.,Ltd.; AS2BM and TiO2P25 from Nippon Aerogil; 100A and 500A from UbeIndustries, Ltd.; Y-LOP from Titan Kogyo K.K.; and sintered products ofthe same. Particular preferable nonmagnetic powders are titanium dioxideand α-iron oxide.

[0065] Based on the objective, an organic powder may be added to thenonmagnetic layer. Examples of such an organic powder are acrylicstyrene resin powders, benzoguanamine resin powders, melamine resinpowders, and phthalocyanine pigments. Polyolefin resin powders,polyester resin powders, polyamide resin powders, polyimide resinpowders, and polyfluoroethylene resins may also be employed.

[0066] [Binder]

[0067] Conventionally known thermoplastic resins, thermosetting resins,reactive resins and mixtures thereof may be employed as binders employedin the magnetic layer and nonmagnetic layer in the present invention.Examples of the thermoplastic resins are polymers and copolymerscomprising structural units in the form of vinyl chloride, vinylacetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters,vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic acidesters, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, andvinyl ether; polyurethane resins; and various rubber resins.

[0068] Further, examples of thermosetting resins and reactive resins arephenol resins, epoxy resins, polyurethane cured resins, urea resins,melamine resins, alkyd resins, acrylic reactive resins, formaldehyderesins, silicone resins, epoxy polyamide resins, mixtures of polyesterresins and isocyanate prepolymers, mixtures of polyester polyols andpolyisocyanates, and mixtures of polyurethane and polyisocyanates. Thethermoplastic resins, the thermosetting resins and the reactive resinsare described in detail in the Handbook of Plastics published by AsakuraShoten.

[0069] Further, when an electron beam-curable resin is employed in themagnetic layer, not only coating strength can be improved to improvedurability, but also the surface is rendered smooth to enhanceelectromagnetic characteristics.

[0070] The above-described resins may be employed singly or incombination. Of these, the use of polyurethane resin is preferred. Inparticular, the use of the following polyurethane resin is furtherpreferred; a polyurethane resin prepared by reacting a cyclic compoundsuch as hydrogenated bisphenol A or a polypropylene oxide adduct ofhydrogenated bisphenol A, a polyol with a molecular weight of 500 to5,000 comprising an alkylene oxide chain, a chain-extending agent in theform of a polyol with a molecular weight of 200 to 500 having a cyclicstructure, and an organic diisocyanate, as well as introducing ahydrophilic polar group; a polyurethane resin prepared by reacting analiphatic dibasic acid such as succinic acid, adipic acid, or sebacicacid, a polyester polyol comprised of an aliphatic diol not having acyclic structure having an alkyl branching side chain such as2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol,2,2-diethyl-1,3-propanediol, a chain-extending agent such as analiphatic diol having a branching alkyl side chain with three or morecarbon atoms, such as 2-ethyl-2-butyl-1,3-propanediol,2,2-diethyl-1,3-propanediol, and an organic diisocyanate, as well asintroducing a hydrophilic polar group; and a polyurethane resin preparedby reacting a cyclic structure such as a dimer diol, a polyol compoundhaving a long alkyl chain, and an organic diisocyanate, as well asintroducing a hydrophilic polar group.

[0071] Examples of polyisocyanates suitable for use in the presentinvention are tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate,hexamethylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate, triphenylmethane triisocyanate, and other isocyanates;products of these isocyanates and polyalcohols; polyisocyanates producedby condensation of isocyanates; and the like. These isocyanates arecommercially available under the following trade names, for example:Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR andMillionate MTL manufactured by Nippon Polyurethane Industry Co. Ltd.;Takenate D-102, Takenate D-110N, Takenate D-200 and Takenate D-202manufactured by Takeda Chemical Industries Co. Ltd.; and Desmodule L,Desmodule IL, Desmodule N and Desmodule HL manufactured by SumitomoBayer Co. Ltd. They can be used singly or in combinations of two or morein each of layers by exploiting differences in curing reactivity.

[0072] The average molecular weight of the polyurethane resin comprisinga polar group that is employed in the present invention desirably rangesfrom 5,000 to 100,000, preferably from 10,000 to 50,000. An averagemolecular weight of equal to or greater than 5,000 is desirable in thatit yields a magnetic coating that does not undergo a decrease inphysical strength, such as by becoming brittle, and that does not affectthe durability of the magnetic recording medium. A molecular weight ofequal to or less than 100,000 does not reduce solubility in solvent andthus affords good dispersion. Further, since the coating materialviscosity does not become high at defined concentrations, manufacturingproperties are good and handling is facilitated.

[0073] Examples of the polar group comprised in the above-describedpolyurethane resins are: —COOM, —SO₃M, —OSO₃M, —P═O(OM)₂, —O—P═O(OM)₂(where M denotes a hydrogen atom or alkali metal base), —OH, —NR₂, —N⁺R₃(where R denotes a hydrocarbon group), epoxy group, —SH, and —CN. Atleast one of these polar groups may be incorporated by copolymerizationor an addition reaction for use. When the polar group-comprisingpolyurethane resin contains an OH group, a branched OH group isdesirable from the perspectives of curing properties and durability. Thebranched OH group number of 2 to 40 is desirably per molecule, with thepresence of 3 to 20 per molecule being preferred. The quantity of suchpolar groups ranges from 10⁻¹ to 10⁻⁸ mol/g, preferably from 10⁻² to10⁻⁶ mol/g.

[0074] Specific examples of binders are VAGH, VYHH, VMCH, VAGF, VAGD,VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and PKFE fromUnion Carbide Corporation; MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF,MPR-TS, MPR-TM, and MPR-TAO from Nisshin Kagaku Kogyo K.K.; 1000W, DX80,DX81, DX82, DX83, and 100FD from Denki Kagaku Kogyo K.K.; MR-104,MR-105, MR110, MR100, MR555, and 400X-110A from Nippon Zeon Co., Ltd.;Nippollan N2301, N2302, and N2304 from Nippon Polyurethane Co., Ltd.;Pandex T-5105, T-R3080, T-5201, Burnock D-400, D-210-80, Crisvon 6109,and 7209 from Dainippon Ink and Chemicals Incorporated.; Vylon UR8200,UR8300, UR-8700, RV530, and RV280 from Toyobo Co., Ltd.; Daipheramine4020, 5020, 5100, 5300, 9020, 9022, and 7020 from Dainichiseika Color &Chemicals Mfg. Co., Ltd.; MX5004 from Mitsubishi Chemical Corporation;Sanprene SP-150 from Sanyo Chemical Industries, Ltd.; and Saran F310 andF210 from Asahi Chemical Industry Co., Ltd.

[0075] In the present invention, the quantity of binder employed in themagnetic layer and nonmagnetic layer desirably falls within a range of 5to 50 weight percent, preferably within a range of 10 to 30 weightpercent, of the ferromagnetic powder (ferromagnetic magnetic powder orferromagnetic hexagonal ferrite powder) or nonmagnetic powder. In thecase of a polyurethane resin, it is desirably employed in a quantity of2 to 20 weight percent, and in the case of polyisocyanate, it isdesirably employed in a quantity of 2 to 20 weight percent. It isdesirable to employ them together. However, for example, when headcorrosion occurs due to the release of trace amounts of chlorine, it ispossible to employ just polyurethane or polyurethane and isocyanate.When another resin in the form of vinyl chloride resin is employed, thedesirable range is 5 to 30 weight percent. When employing polyurethanein the present invention, the glass transition temperature ranges from−50 to 150° C., preferably from 0 to 100° C. The elongation at breakdesirably ranges from 100 to 2,000 percent, the stress at break from0.49 to 98 MPa (0.05 to 10 kg/mm²), and the yield point from 0.49 to 98MPa (0.05 to 10 kg/mm²).

[0076] The magnetic recording medium of the present invention comprisesa nonmagnetic layer and at least one magnetic layer. Accordingly, thequantity of binder; the proportion of vinyl chloride resin, polyurethaneresin, polyisocyanate, or some other resin in the binder; the molecularweight and quantity of polar groups in the various resins in themagnetic layer; and the physical characteristics of the above-describedresins may be varied as needed from the nonmagnetic layer to theindividual magnetic layers. They should be optimized for each layer.Known techniques for a multilayered magnetic layer may be applied. Forexample, when varying the quantity of binder in each layer, the quantityof binder in the magnetic layer may be increased to effectively reducerubbing damage to the magnetic layer surface, and the quantity of binderin the nonmagnetic layer may be increased to impart flexibility for goodhead touch.

[0077] [Other Additives]

[0078] As needed, additives can be added to the magnetic layer ornonmagnetic layer in the present invention. Examples of additives areabrasives, lubricants, dispersion agents, dispersion assistant agents,fungicides, antistatic agents, antioxidatnts, solvents, carbon black andthe like.

[0079] Examples are molybdenum disulfide; tungsten disulfide; graphite;boron nitride; graphite fluoride; silicone oils; silicones having apolar group; fatty acid-modified silicones; fluorine-containingsilicones; fluorine-containing alcohols; fluorine-containing esters;polyolefins; polyglycols; polyphenyl ethers; aromatic ring-containingorganic phosphorous acids such as phenylphosphorous acid and theiralkali metal salts; alkylphosphorous acids such as octylphosphorous acidand their alkali metal salt; aromatic phosphoric acid esters such asphenylphosphate and their alkali metal salts; alkylphosphoric acidesters such as octylphosphate and their alkali metal salt; alkylsulfonicacid esters and their alkali metal salts; fluorine-containingalkylsulfuric acid esters and their alkali metal salts; monobasic fattyacids with 10 to 24 carbon atoms (which may contain an unsaturated bondor be branched) such as lauric acid and their alkali metal salts;monofatty esters, difatty esters, or polyfatty esters such as butylstearate comprising a monobasic fatty acid having 10 to 24 carbon atoms(which may contain an unsaturated bond or be branched) and any one fromamong a monohydric, dihydric, trihydric, tetrahydric, pentahydric orhexahydric alcohol having 2 to 22 carbon atoms (which may contain anunsaturated bond or be branched); alkoxy alcohols having 12 to 22 carbonatoms (which may contain an unsaturated bond or be branched) andmonoalkyl ethers of alkylene oxide polymers; fatty acid amidescomprising 2 to 22 carbon atoms, and fatty acid amines comprising 8 to22 carbon atoms. Compounds comprising alkyl groups, aryl groups, andaralkyl groups substituted with groups other than the above-mentionedhydrocarbon groups such as nitro groups or hydrocarbon groups containinghalogens such as F, Cl, Br, CF₃, CCl₃, and CBr₃ may also be employed.Further, nonionic surfactants such as alkylene oxid-based one,glycerine-based one, glycidol-based one and alkyl phenol ethylene oxideadducts; cationic surfactants such as cyclic amines, ester amides,quaternary ammonium salts, hydantoin derivatives, heterocycles,phosphoniums, and sulfoniums; anionic surfactants such as carboxylicacids, sulfonic acids, sulfuric esters, and other acid group-comprisingcompounds; and amphoteric surfactants such as amino acids, aminosulfonicacids, sulfuric and phosphoric acid esters of aminoalcohols, and alkylbetaines may also be employed. These surfactants are described in detailin, “A Handbook of Surfactants” (published by Sangyo Tosho K.K.). Theseadditives need not necessarily be pure, and may comprise isomers,unreacted products, side-products, decomposition products, oxides, andother impurities in addition to the principal components. The impuritiesdesirably constitute equal to or less than 30 weight percent, preferablyequal to or less than 10 weight percent.

[0080] Specific examples of these additives are: NAA-102, hydrogenatedcastor oil fatty acid, NAA-42, Cation SA, Nymeen L-201, Nonion E-208,Anon BF and Anon LG manufactured by NOF Corporation; FAL-205 and FAL-123manufactured by Takemoto Oil & Fat Co.,Ltd.; NJLUB OL manufactured byNew Japan Chemical Co.Ltd.; TA-3 manufactured by Shin-Etsu ChemicalCo.Ltd.; Armide P manufactured by Lion Armour Co.,Ltd.; Duomine TDOmanufactured by Lion Corporation; BA-41G manufactured by Nisshin OilMills, Ltd.; and Profan 2012E, Newpole PE61 and Ionet MS-400manufactured by Sanyo Chemical Industries, Ltd.

[0081] Further, in the present invention, carbon black may be admixed tothe magnetic layer and nonmagnetic layer to decrease surface resistivityand achieve the desired micro Vicker's hardness. The micro Vicker'shardness normally ranges from 25 to 60 kg/mm² (0.25 to 0.59 GPa), andpreferably from 30 to 50 kg/mm² (0.29 to 0.49 GPa) to adjust head touch.It can be measured with a thin-film hardness meter (the HMA-400manufactured by NEC Corporation) using a triangular diamond indenter tipwith a front end radius of 0.1 μm and an edge angle of 80 degrees.Examples of carbon blacks suitable for use in the magnetic layer and thenonmagnetic layer are furnace black for rubber, thermal for rubber,black for coloring, and acetylene black. The specific surface areadesirably ranges from 5 to 500 m²/g, the DBP oil absorption capacityfrom 10 to 400 mL/100 g, the particle diameter from 5 to 300 nm, the pHfrom 2 to 10, the moisture content from 0.1 to 10 percent, and the tapdensity from 0.1 to 1 g/mL.

[0082] Specific examples of types of carbon black suitable for use inthe magnetic and nonmagnetic layers in the present invention are: BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700 and VULCAN XC-72 from CabotCorporation; #80, #60, #55, #50 and #35 from Asahi Carbon Co., Ltd.;#3050B, #3150B, #3250B, #3750B, #3950B, #2400B, #2300, #1000, #970B,#950, #900, #850B, #650B, #30, #40, #10B and MA-600 from MitsubishiChemical Corporation; CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250,3500, 2100, 2000, 1800, 1500, 1255, 1250, 150, 50, 40, 15 and RAVEN-MT-Pfrom Columbia Carbon Co., Ltd.; and Ketjen Black EC from Lion Akzo Co.,Ltd.

[0083] The carbon black employed can be surface treated with adispersing agent or the like, grafted with a resin, or a portion of thesurface may be graphite-treated. Further, the carbon black may bedispersed with a binder prior to being added to the magnetic ornonmagnetic coating material. These types of carbon black may beemployed singly or in combination. When employing carbon black, thequantity preferably ranges from 0.1 to 30 weight percent with respect tothe weight of the magnetic material. In the magnetic layer, carbon blackworks to prevent static, reduce the coefficient of friction, impartlight-blocking properties, enhance film strength, and the like; theproperties vary with the type of carbon black employed. Accordingly, thetype, quantity, and combination of carbon blacks employed in the presentinvention may be determined separately for the magnetic layer and thenonmagnetic layer based on the objective and the various characteristicsstated above, such as particle size, oil absorption capacity, electricalconductivity, and pH, and be optimized for each layer. The Carbon BlackHandbook compiled by the Carbon Black Association may be consulted fortypes of carbon black suitable for use in the magnetic layer of thepresent invention.

[0084] In the present invention, known organic solvent can be employed.The organic solvent employed in the present invention may be used in anyratio. Examples are ketones such as acetone, methyl ethyl ketone, methylisobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, andtetrahydrofuran; alcohols such as methanol, ethanol, propanol, butanol,isobutyl alcohol, isopropyl alcohol, and methylcyclohexanol; esters suchas methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate,ethyl lactate, and glycol acetate; glycol ethers such as glycol dimethylether, glycol monoethyl ether, and dioxane; aromatic hydrocarbons suchas benzene, toluene, xylene, cresol, and chlorobenzene; chlorinatedhydrocarbons such as methylene chloride, ethylene chloride, carbontetrachloride, chloroform, ethylene chlorohydrin, and dichlorobenzene;N,N-dimethylformamide; and hexane. These organic solvents need not be100 percent pure and may contain impurities such as isomers, unreactedmaterials, by-products, decomposition products, oxides and moisture inaddition to the main components. The content of these impurities ispreferably equal to or less than 30 percent, more preferably equal to orless than 10 percent. Preferably the same type of organic solvent isemployed in the present invention in the magnetic layer and in thenonmagnetic layer. However, the amount added may be varied. Thestability of coating is increased by using a solvent with a high surfacetension (such as cyclohexanone or dioxane) in the nonmagnetic layer.Specifically, it is important that the arithmetic mean value of theupper layer solvent composition be not less than the arithmetic meanvalue of the nonmagnetic layer solvent composition. To improvedispersion properties, a solvent having a somewhat strong polarity isdesirable. It is desirable that solvents having a dielectric constantequal to or higher than 15 are comprised equal to or higher than 50percent of the solvent composition. Further, the dissolution parameteris desirably 8 to 11.

[0085] Different types and quantities of dispersants, lubricants, andsurfactants may be employed as necessary in the magnetic layer andnonmagnetic layer in the present invention. The examples given here arenot to be construed as limits. For example, a dispersant impartsadsorptive or bonding properties through polar groups, adsorbing orbinding by means of polar groups chiefly to the surface of theferromagnetic powder in the magnetic layer and chiefly to the surface ofthe nonmagnetic powder in the nonmagnetic layer. It is thought that oncean organic phosphorus compound has been adsorbed, it tends not to desorbfrom the surface of a metal or metallic compound. Accordingly, since thesurface of the ferromagnetic powder (ferromagnetic metal powder andferromagnetic hexagonal ferrite powder) or the surface of thenonmagnetic powder in the present invention is coated with alkyl groups,aromatic groups, or the like, affinity of the ferromagnetic powder ornonferromagnetic powder for the binder resin component increases and thedispersion stability of the ferromagnetic powder or nonmagnetic powderimproves. Further, since lubricants are present in a free state, it isconceivable to employ fatty acids having different melting points in thenonmagnetic layer and magnetic layer to control seepage out onto thesurface, employ esters of different melting points and polarity tocontrol seepage out onto the surface, adjust the quantity of surfactantto improve the stability of the coating, and increase the quantity oflubricant in the nonmagnetic layer to improve the lubricating effect.Further, all or a portion of the additives employed in the presentinvention may be added during any step during the manufacturing of thecoating liquid employed for the magnetic layer or nonmagnetic layer. Forexample, there are cases where additives are admixed with theferromagnetic powder prior to the kneading step, cases where they areadded during the step of kneading the ferromagnetic powder, binder, andsolvent, cases where they are added during the dispersion step, caseswhere they are added following dispersion, and cases where they areadded immediately prior to coating.

[0086] [Backcoat Layer and Adhesion-enhancing Layer]

[0087] Generally, greater repeat running properties are demanded ofmagnetic tapes employed in computer data recording than of audio andvideo tapes. To maintain such high running durability, a backcoat layercan be provided on the reverse side of the nonmagnetic support from theside on which the nonmagnetic layer and magnetic layer are provided. Thebackcoat layer coating liquid can be prepared by dispersing the binderand granular components such as abrasives and antistatic agents in anorganic solvent. Various inorganic pigments and carbon black may beemployed as granular components. Nitrocellulose, phenoxy resin, vinylchloride resin, polyurethane, and other resins may be employed singly orin combination as the binder.

[0088] An undercoating layer may be provided on the surface coated witha magnetic layer coating liquid and a backcoat layer coating liquid toincrease the adhesive strength between the nonmagnetic support and themagnetic layer and/or the nonmagnetic lower layer and a backcoat layer,and to enhance the smoothness of the magnetic layer and backcoat layersurfaces.

[0089] A solvent-soluble substance may be employed as a binder in theundercoating layer: such as polyester resin, polyamide resin,polyamidoimide resin, polyurethane resin, vinyl chloride resin,vinylidene chloride resin, phenol resin, epoxy resin, urea resin,melamine resin, formaldehyde resin, silicone resin, starch,modified-starch resin, alginic acid compounds, casein, gelatin,pullulan, dextran, chitin, chitosan, rubber latex, gum Arabic, funori,natural gum, dextrin, modified cellulose resin, polyvinyl alcohol resin,polyethylene oxide, polyacrylic acid-based resin, polyvinyl pyrrolidone,polyethyleneimine, polyvinyl ether, polymaleic acid copolymers,polyacrylamide, and alkyd resins.

[0090] The undercoating layer is not specifically limited other thanthat it be from 0.01 to 3.0 μm in thickness, with a thickness of 0.05 to2.0 μm being preferred and thickness of 0.1 to 1.5 μm being of evengreater preference. The glass transition temperature of the resinemployed in the undercoating layer desirably ranges from 30 to 120° C.,and preferably from 40 to 80° C. Blocking does not occur along edgesurfaces at equal to or greater than 0° C., and at equal to or less than120° C., internal stress in the smooth layer is moderated and adhesivestrength is good.

[0091] [The Layer Structure]

[0092] In the magnetic recording medium of the present invention, atleast two layers of coating film, that is, a nonmagnetic layer and amagnetic layer over the nonmagnetic layer, are provided on at least onesurface of a nonmagnetic support. The magnetic layer may comprise two ormore layers as needed. Further, a backcoat layer is provided as neededon the surface of the reverse side of the nonmagnetic support. Stillfurther, lubricating coated films and various coated films forprotecting the magnetic layer may be provided as needed on the magneticlayer in the magnetic recording medium of the present invention. Stillfurther, undercoating layers (adhesion-enhancing layers) may be providedbetween the nonmagnetic support and nonmagnetic layers to improveadhesion between coated films and the nonmagnetic support.

[0093] A magnetic layer and a nonmagnetic layer may be provided on oneor both sides of the nonmagnetic support in the magnetic recordingmedium of the present invention. The nonmagnetic layer (lower layer) andmagnetic layer (upper layer) may be provided in such a manner that thelower layer is applied first, with the upper layer being applied whilethe lower layer is still wet (W/W), or the lower layer may be driedbefore applying the upper magnetic layer (W/D). Simultaneous orsequential wet coating is preferred from the perspective of productionefficiency, but in the case of disks, coating following drying is fullypossible. In the multilayer configuration of the present invention,since the upper layer and lower layer can be simultaneously formed bysimultaneous or sequential wet coating (W/W), a surface treatment stepsuch as calendering can be effectively utilized to improve the surfaceroughness of the upper magnetic layer even in the case of ultrathinlayers.

[0094] In the present invention, the thickness of the nonmagneticsupport desirably ranges from 3 to 80 μm. In computer tapes, anonmagnetic support having a thickness of 3.5 to 7.5 μm, preferably from3 to 7 μm, can be employed. Further, when providing an undercoatinglayer between the nonmagnetic support and a nonmagnetic layer or amagnetic layer, the thickness of the undercoating layer is desirablyfrom 0.01 to 0.8 μm, preferably from 0.02 to 0.6 μm. Further, whenproviding a backcoat layer on the reverse side from the side on whichthe nonmagnetic layer and the magnetic layer is provided on thenonmagnetic support, the thickness thereof is desirably from 0.1 to 1.0μm, preferably from 0.2 to 0.8 μm.

[0095] The thickness of the magnetic layer is optimized based on thesaturation magnetization level and head gap length of the magnetic heademployed and the recording signal band, but is generally from 10 to 100nm, preferably from 20 to 80 nm, and more preferably from 30 to 80 nm.Further, the thickness fluctuation rate of the magnetic layer isdesirably within ±50 percent, preferably within ±40 percent. Themagnetic layer comprises at least one layer, but may be separated intotwo or more layers having different magnetic characteristics. Knownmultilayer magnetic layer configurations may be employed.

[0096] The thickness of the nonmagnetic layer is desirably 0.02 to 3.0μm, preferably from 0.05 to 2.5 μm, and more preferably, from 0.1 to 2.0μm. In the magnetic recording medium of the present invention, thenonmagnetic layer can effectively function so long as it is essentiallynonmagnetic. For example, even when an impurity or an intentional traceamount of magnetic material is contained, the effect of the presentinvention is exhibited and the configuration can be seen as beingessentially identical to that of the magnetic recording medium of thepresent invention. The term “essentially identical” means that theresidual magnetic flux density of the nonmagnetic layer is equal to orless than 10 T·m (100 G) or the coercive force is equal to or less than7.96 kA/m (100 Oe), with the absence of a residual magnetic flux densityand coercive force being preferred.

[0097] [Physical Characteristics]

[0098] In the magnetic recording medium of the present invention, thesaturation magnetic flux density of the magnetic layer is desirably from100 to 300 T·m (1,000 to 3,000 G). The coercive force (Hr) of themagnetic layer is desirably from 143.3 to 318.4 kA/m (1,800 to 4,000Oe), preferably from 159.2 to 278.6 kA/m (2,000 to 3,500 Oe). Thecoercive force distribution is desirably narrow, with the SFD and SFDrbeing equal to or less than 0.6, preferably equal to or less than 0.2.

[0099] The coefficient of friction of the magnetic recording medium ofthe present invention with the head is desirably equal to or less than0.5, preferably equal to or less than 0.3, over a temperature range of−10 to 40° C. and a humidity range of 0 to 95 percent. Specific surfaceresistivity is preferably from 10⁴ to 10¹² Ω/sq on the magnetic surface,and the charge potential is desirably within a range of −500 to +500 V.The modulus of elasticity at 0.5 percent elongation of the magneticlayer is desirably from 0.98 to 19.6 GPa (100 to 2,000 kg/mm²) in allin-plane directions. The breaking strength is desirably from 98 to 686MPa (10 to 70 kg/mm²). The modulus of elasticity of the magneticrecording medium is desirably from 0.98 to 14.7 GPa (100 to 1,500kg/mm²) in all in-plane directions. The residual elongation is desirablyequal to or less than 0.5 percent. The thermal shrinkage rate at anytemperature equal to or less than 100° C. is desirably equal to or lessthan 1 percent, preferably equal to or less than 0.5 percent, and morepreferably equal to or less than 0.1 percent.

[0100] The glass transition temperature of the magnetic layer (the peakloss elastic modulus of dynamic viscoelasticity measured at 110 Hz) isdesirably from 50 to 180° C., and that of the nonmagnetic layer isdesirably from 0 to 180° C. The loss elastic modulus desirably fallswithin a range of 1×10⁷ to 8×10⁸ Pa (1×10⁸ to 8×10⁹ dyne/cm²) and theloss tangent is desirably equal to or less than 0.2. Excessive high losstangent tends to cause a adhesion failure. These thermal and mechanicalcharacteristics are desirably identical to within 10 percent in allin-plane directions of the medium.

[0101] The residual solvent contained in the magnetic layer is desirablyequal to or less than 100 mg/m², preferably equal to or less than 10mg/m². The void rate of the coated layer is desirably equal to or lessthan 30 volume percent, preferably equal to or less than 20 volumepercent, in both the nonmagnetic and magnetic layers. A low void rate isdesirable to achieve high output, but there are objectives for whichensuring a certain value is good. For example, in disk media in whichrepeat applications are important, a high void rate is often desirablefor running durability.

[0102] The maximum height SR_(max) of the magnetic layer is desirablyequal to or less than 0.5 μm. The ten-point average roughness SRz isdesirably equal to or less than 0.3 μm. The center surface peak SRp isdesirably equal to or less than 0.3 μm. The center surface valley depthSRv is desirably equal to or less than 0.3 μm. The center surfacesurface area SSr is desirably from 20 to 80 percent. And the averagewavelength S λ a is desirably from 5 to 300 μm. These can be readilycontrolled by controlling the surface properties by means of fillersemployed in the support and the surface shape of the rolls employed incalendering. Curling is desirably within ±3 mm.

[0103] In the magnetic recording medium of the present invention, it ispossible to vary the physical characteristics between the nonmagneticlayer and the magnetic layer based on the objective. For example, whileincreasing the modulus of elasticity of the magnetic layer to improverunning durability, it is possible to make the modulus of elasticity ofthe nonmagnetic layer lower than that of the magnetic layer to enhancecontact between the magnetic recording medium and the head.

[0104] [Manufacturing Method]

[0105] The process of manufacturing the magnetic layer coating liquidand nonmagnetic layer coating liquid of the magnetic recording medium ofthe present invention comprises at least a kneading step, dispersionstep, and mixing steps provided as needed before and after these steps.Each of the steps may be divided into two or more stages. All of thestarting materials employed in the present invention, including theferromagnetic hexagonal ferrite powder or ferromagnetic metal powder,nonmagnetic powder, benzenesulfonic acid derivatives, π electronconjugate-type conductive polymers, binder, carbon black, abrasives,antistatic agents, lubricants, and solvents may be added at thebeginning or during any step. Further, each of the starting materialsmay be divided and added during two or more steps. For example,polyurethane may be divided up and added during the kneading step,dispersion step, and mixing step for viscosity adjustment followingdispersion. To achieve the object of the present invention,conventionally known manufacturing techniques may be employed for someof the steps. A kneading device of high kneading strength such as anopen kneader, continuous kneader, pressure kneader, or extruder isdesirably employed in the kneading step. When a kneader is employed, allor a portion (with equal to or greater than 30 percent of the totalbinder being desirable) of the magnetic powder or nonmagnetic powder andbinder can be kneaded in a proportion of 15 to 500 parts by weight per100 parts by weight of magnetic material. The details of the kneadingprocess are described in detail in Japanese Unexamined PatentPublication (KOKAI) Heisei Nos. 1-106338 and 1-79274. Further, glassbeads may be employed to disperse the magnetic layer coating liquid andnonmagnetic coating liquid. A dispersion medium having a high specificgravity such as zirconia beads, titania beads, or steel beads issuitable for use as the glass beads. The particles diameter and fillrate of the dispersion medium are optimized for use. A known dispersingmachine may be employed.

[0106] In the method of manufacturing the magnetic recording medium ofthe present invention, the magnetic layer coating liquid can be coatedto a prescribed film thickness on a nonmagnetic layer that has beenprovided on a nonmagnetic support, to form a magnetic layer. In thisprocess, multiple magnetic layer coating liquids can be sequentially orsimultaneously multilayer coated, and the nonmagnetic layer coatingliquid and magnetic layer coating liquid can be sequentially orsimultaneously multilayer coated. Coating machines suitable for use incoating the magnetic and nonmagnetic coating materials mentioned aboveare air doctor coaters, blade coaters, rod coaters, extrusion coaters,air knife coaters, squeeze coaters, immersion coaters, reverse rollcoaters, transfer roll coaters, gravure coaters, kiss coaters, castcoaters, spray coaters, spin coaters, and the like. For example, “RecentCoating Techniques” (May 31, 1983), issued by the Sogo Gijutsu CenterK.K. may be referred to in this regard.

[0107] In the case of a magnetic tape, the layer formed by coating themagnetic layer coating liquid is magnetically oriented in thelongitudinal direction using a cobalt magnet or solenoid on theferromagnetic powder contained in the layer formed by coating themagnetic layer coating liquid. In the case of a disk, although isotropicorientation can be adequately achieved without orientation using anorientation device, the positioning of cobalt magnets at mutuallyoblique angles or the use of a known random orientation device such asthe application of an alternating current magnetic field with solenoidsis desirably employed. In the case of ferromagnetic metal powder, theterm “isotropic orientation” generally desirably means two-dimensionalin-plane randomness, but can also mean three-dimensional randomness whena vertical component is imparted. In the case of hexagonal ferrite,three-dimensional randomness in the in-plane and vertical directions isgenerally readily achieved, but two-dimensional in-plane randomness isalso possible. A known method such as magnets with opposite polesopposed may be employed to impart isotropic magnetic characteristics ina circumferential direction using a vertical orientation. Verticalorientation is particularly desirable in the case of high-densityrecording. Further, spin coating may be employed to achievecircumferential orientation.

[0108] The temperature and flow rate of drying air and the coating rateare desirably determined to control the drying position of the coatedfilm. The coating rate is desirably from 20 m/min to 1,000 m/min and thetemperature of the drying air is desirably equal to or greater than 60°C. It is also possible to conduct suitable predrying before entry intothe magnet zone.

[0109] Following drying, a surface smoothing treatment is applied to thecoated layer. For example, supercalender rolls are employed in thesurface smoothing treatment. The surface smoothing treatment eliminatesholes produced by the removal of solvent during drying and improves thefill rate of ferromagnetic powder in the magnetic layer, making itpossible to obtain a magnetic recording medium of high electromagneticcharacteristics. Heat-resistant plastic rolls such as epoxy, polyimide,polyamide, and polyamidoimide rolls may be employed as the calenderingrolls. Processing with metal rolls is also possible. The magneticrecording medium of the present invention desirably has an extremelysmooth surface such that the center surface average roughness is 0.1 to4 nm, preferably 1 to 3 nm at a cutoff value of 0.25 mm. For example,this is achieved by subjecting a magnetic layer formed by selecting aspecific ferromagnetic powder and binder such as have been set forthabove to the above-described calendering. Calendering is desirablyconducted under conditions of a calendering roll temperature fallingwithin a range of 60 to 100° C., preferably within a range of 70 to 100°C., and more preferably within a range of 80 to 100° C., at a pressurefalling within a range of 100 to 500 kg/cm, preferably within a range of200 to 450 kg/cm, and more preferably within a range of 300 to 400kg/cm.

[0110] Means of reducing the thermal shrinkage rate include heattreatment in a web-shape while handling at low tension and heattreatment (thermo processing) with the tape in bulk or in a stackedstate such as wound on a cassette. Both may be employed; from theperspective of achieving a magnetic recording medium of high output andlow noise, thermo processing is desirable.

[0111] The magnetic recording medium obtained can be cut to desired sizewith a cutter or the like for use.

EMBODIMENTS

[0112] The present invention is described in greater detail belowthrough embodiments. The components, proportions, operations, sequences,and the like indicated in the embodiments can be modified withoutdeparting from the spirit or scope of the present invention, and are notto be construed as being limited to the embodiments below. Further,unless specifically indicated otherwise, the “parts” indicated in theembodiments refer to parts by weight.

Preparation Example 1

[0113] A 4 g quantity of montmorillonite (Kunipia G made by KunimineKogyo K.K.) was dispersed in 200 mL of water. To this, 2.5 g ofn-dodecyltrimethylammonium chloride were added, and the components werestirred for 1 hour at room temperature. The mixture was filtered bybeing drawn through a membrane filter while being thoroughly washed withwater, and then dried under vacuum for 24 hours at 100° C., yieldingmontmorillonite (filler A) coated with an organic ammonium salt.

[0114] Separately, 2-chloroparaphenylenediamine in a quantitycorresponding to 90 molar percent and 4,4′-diaminodiphenylether in aquantity corresponding to 10 molar percent were dissolved in dehydratedN-methyl pyrrolidone. The above-described montmorillonite coated with anorganic compound was dispersed in a quantity of 3 weight percentrelative to the aromatic polyamide obtained. To this mixture,2-chloroterephthalic acid chloride was added in a quantity correspondingto 98.5 molar percent, polymerization was conducted for 2 hours withstirring, and the reaction solution was neutralized with lithiumcarbonate, yielding an aromatic polyamide solution with a polymerconcentration of 11 weight percent.

[0115] This solution was fed to the cap by an extruder and flowed andextended onto a stainless-steel belt with a mirror surface. The extendedpolymer solution was first blown with 160° C. hot air, and then with180° C. hot air for one minute each to evaporate the solvent. Afterbeing peeled off, the film was longitudinally stretched 1.15-fold. Next,the film was passed through a water vat for 2 min to extract with waterthe residual solvent and inorganic salts produced by neutralization.Next, the film was transversely stretched 1.45-fold and heat treated ina tenter at a temperature of 280° C. under hot air at an air flow rateof 5 m/sec. This yielded an aromatic polyamide film (Aramid-A) with atotal thickness of 6 μm.

Preparation Example 2

[0116] A 1.6 mol quantity of ethylene glycol was admixed per mol ofterephthalic acid, and filler A that had been coated with an organiccompound was added in a proportion of three parts by weight per 100parts by weight of the polymer being theoretically produced. A reactionwas conducted for 2 hours at 255° C. to produce an oligomer comprisedprimarily of bishydroxyethyl terephthalate. Subsequently, a catalyst inthe form of antimony trioxide was added in a molar proportion of 100 ppmrelative to the terephthalic acid, and a reaction was conducted for 4hours at 275° C. under reduced pressure. Polymer melt was dischargedthrough the collection outlet. This was cooled and cut into pellets.

[0117] The pellets were dried for 10 hours at 80° C., charged to anextruder, melted by heating to 270° C., extruded in sheet form throughthe orifice of a T-die, wound onto a cooling drum with a surfacetemperature of 10° C., and cooled to prepare an unstretched film 600 μmin thickness. Next, the unstretched film was longitudinally stretchedthree-fold with rolls at a stretching temperature of 90° C.,transversely stretched 3.3-fold with a tenter, and heat treated at 230°C. at 5 percent relaxation, yielding a polyethylene terephthalate film(PET-A) 60 μm in thickness. Preparation of magnetic layer coating liquidFerromagnetic acicular metal powder 100 parts Composition: Fe/Co/Al/Y =68/20/7/5 Surface treatment agent: Al₂O₃, Y₂O₃ Coercive force (Hc): 199kA/m (2,500 Oe) Crystallite size: 130 Å Major axis diameter: 65 nmAcicular ratio: 6 Specific surface area by BET method: 46 m²/gSaturation magnetization (σs): 150 m²/kg (150 emu/g) Polyurethane resin12 parts Branched side chain-containing polyester polyol/diphenylmethane diisocyanate type, Hydrophilic polar group: —SO₃Nacontent is 70 eq/ton. Phenylphosphorous acid 3 parts α-Al₂O₃ (particlesize: 0.15 μm) 2 parts Carbon black (particle size: 20 nm) 2 partsCyclohexanone 110 parts Methyl ethyl ketone 100 parts Toluene 100 partsButyl stearate 2 parts Stearic acid 1 part Preparation of nonmagneticlayer coating liquid Nonmagnetic inorganic powder 85 parts α-iron oxideSurface treatment agent: Al₂O₃, SiO₂ Major axis diameter: 0.15 μm Tapdensity: 0.8 g/ml Acicular ratio: 7 Specific surface area by BET method:52 m²/g pH: 8 DBP oil absorption capacity: 33 g/100 g Carbon black 20parts DBP oil absorption capacity: 120 ml/100 g pH: 8 Specific surfacearea by BET method: 250 m²/g Volatile content: 1.5 percent Polyurethaneresin 12 parts Branched side chain-containing polyester polyol/diphenylmethane diisocyanate type, Hydrophilic polar group: —SO₃Nacontent is 70 eq/ton. Acrylic resin 6 parts Benzylmethacrylate/diacetone acrylamide type, Hydrophilic polar group: —SO₃Nacontent is 60 eq/ton. Phenylphosphorous acid 3 parts α-Al₂O₃ (meanparticle diameter: 0.2 μm) 1 part Cyclohexanone 140 parts Methyl ethylketone 170 parts Butyl stearate 2 parts Stearic acid 1 part

[0118] The individual components of the above-described composition ofthe magnetic layer coating liquid and composition of the nonmagneticlayer coating liquid were kneaded for 60 min in an open kneader and thendispersed for 120 min in a sand mill. Six parts of trifunctionallow-molecular-weight polyisocyanate compound (Coronate 3041 made byNippon Polyurethane Co., Ltd.) were added to the dispersions obtained,mixing was conducted for a further 20 min with stirring, and themixtures were filtered through a filter having an average pore diameterof 1 μm to prepare a magnetic layer coating liquid and a nonmagneticlayer coating liquid. The above-described nonmagnetic layer coatingliquid was then coated in a quantity calculated to yield a dry thicknessof 1.8 μm, and immediately thereafter, the above-described magneticlayer coating liquid was coated in a quantity calculated to yield a drythickness of 0.2 μm by a simultaneous multilayer coating on the aromaticpolyamide support (Aramid-A) prepared in Preparation Example 1. Whilethe two layers were still wet, magnetic orientation was conducted with a300 T·m (3000 gauss) magnet, the layers were dried, surface smoothingtreatment was conducted at 90° C. at a linear pressure of 300 kg/cm at arate of 100 m/min with a seven-stage calender comprised solely of metalrolls, a heat treatment was conducted at 70° C. for 48 hours, and thefilm was slit to a ½-inch width to prepare magnetic tape.

Embodiments 2 to 4

[0119] With the exception that the type of clay mineral added during thepreparation of the nonmagnetic support was changed, as indicated inTable 1, magnetic tapes were prepared by the same method as inEmbodiment 1.

Comparative Examples 1 and 2

[0120] With the exception that the type of filler added during thepreparation of the nonmagnetic support or the size of the magneticmaterial was changed, as indicated in Table 1, magnetic tapes wereprepared by the same method as in Embodiment 1.

Embodiment 5

[0121] Preparation of magnetic layer coating liquid Ferromagneticplate-shaped hexagonal ferrite powder 100 parts Composition (molarratio): Ba/Fe/Co/Zn = 1/11/0.2/0.8 Coercive force (Hc): 195 kA/m (2,450Oe) Plate diameter: 26 nm Plate ratio: 3 Specific surface area by BETmethod: 50 m²/g Saturation magnetization (σs): 60 A · m²/kg (60 emu/g)Polyurethane resin 12 parts Branched side chain-containing polyesterpolyol/ diphenylmethane diisocyanate type, Hydrophilic polar group:—SO₃Na content is 70 eq/ton. Phenylphosphorous acid 3 parts α-Al₂O₃(particle size: 0.15 μm) 2 parts Carbon black (particle size: 20 nm) 2parts Cyclohexanone 110 parts Methyl ethyl ketone 100 parts Toluene 100parts Butyl stearate 2 parts Stearic acid 1 part

[0122] Each component of the above-described magnetic layer coatingliquid was prepared by the same method as in Embodiment 1 to obtain amagnetic layer coating liquid. A nonmagnetic coating liquid identical tothat in Embodiment 1 was coated in a quantity calculated to yield a drythickness of 1.8 μm, and immediately thereafter, the above-describedmagnetic layer coating liquid was coated in a quantity calculated toyield a dry thickness of 0.2 μm by a simultaneous multilayer coating onthe polyethylene terephthalate support (PET-A) prepared in PreparationExample 2. While the two layers were still wet, random orientation wasconducted by passing the layers through two field intensity alternatingcurrent magnetic field generators with a frequency of 50 Hz and amagnetic field intensity of 25 T·m (250 gauss) and a frequency of 50 Hzand a magnetic field intensity of 12 T·m (120 gauss), respectively.Following drying, the film was treated with a seven-stage calender at atemperature of 90° C. and linear pressure of 300 kg/cm, and a heattreatment was conducted at 70° C. for 48 hours. The film was thenpunched to 3.7 inches, surface polished, and inserted into a 3.7-inchZip-disk cartridge provided with internal liner. Prescribed mechanicalparts were then added to obtain a 3.7-inch flexible disk.

Embodiments 6 to 8

[0123] With the exception that the type of clay mineral added during thepreparation of the nonmagnetic support was changed, as indicated inTable 2, magnetic disks were prepared by the same method as inEmbodiment 5.

Comparative Examples 3 and 4

[0124] With the exception that the type of filler added during thepreparation of the nonmagnetic support or the size of the magneticmaterial was changed, as indicated in Table 2, magnetic disks wereprepared by the same method as in Embodiment 5.

[0125] Measurement of the Error Rate (Initial and at Elevated Humidityand Temperature)

[0126] A recording signal was recorded on a magnetic tape in 8-10conversion PR1 equalization mode and on a flexible disk in (2,7) RLLmodulation mode at a temperature of 23° C. at 50 percent relativehumidity, and the various measurements were conducted under environmentsof 23° C. and 50 percent RH, and 50° C. and 80 percent RH. TABLE 1Magnetic Magnetic Type of Major axis length Crystallite materialcomposition material characteristics Error rate nonmagnetic of magneticmaterial size Co Al Y Hc σs S_(BET) 23° C., 50% 50° C., 80% support nm AAT % AT % AT % kA/m A · m²/kg m²/g ×10⁻⁵ ×10⁻⁵ Embodiment 1 Aramid-A 60130 20 7 5 199 145 48 0.02 0.06 Embodiment 2 Aramid-B 60 130 20 7 5 199145 48 0.03 0.09 Embodiment 3 Aramid-C 60 130 20 7 5 199 145 48 0.040.04 Embodiment 4 Aramid-D 60 130 20 7 5 199 145 48 0.04 0.05 Comp. Ex.1 Aramid-e 60 130 20 7 5 199 145 48 0.06 1.81 Comp. Ex. 2 Aramid-A 120 200 20 7 5 199 145 48 1.60 1.69

[0127] TABLE 2 Magnetic Magnetic Type of Plate diameter materialcomposition: material characteristics Error rate nonmagnetic of magneticmolar ratio (Ba = 1) Hc σs S_(BET) 23° C., 50% 50° C., 80% supportmaterial (nm) Fe Co Zn kA/m A · m²/kg m²/g ×10⁻⁵ ×10⁻⁵ Embodiment 5PET-A 26 9.0 0.2 0.8 191 60 50 0.35 0.42 Embodiment 6 PET-B 26 9.0 0.20.8 191 60 50 0.45 0.54 Embodiment 7 PET-C 26 9.0 0.2 0.8 191 60 50 0.550.62 Embodiment 8 PET-D 26 9.0 0.2 0.8 191 60 50 0.50 0.52 Comp. Ex. 3PET-e 26 9.0 0.2 0.8 191 60 50 0.42 2.50 Comp. Ex. 4 PET-A 50 9.0 0.20.8 191 60 50 14.80 15.01

[0128] Evaluation Results

[0129] The magnetic tapes of Embodiments 1 to 4, comprisingferromagnetic metal powder with an average major axis length of 20 to100 nm in the magnetic layer and a clay mineral treated with an organicmaterial in the nonmagnetic support, all had low error rates. Further,there was little change in the error rate with variation in conditionsof temperature and humidity. This was attributed to low variation inexpansion coefficients based on the temperature and humidity conditionsof the nonmagnetic support. Further, the magnetic tapes of Embodiments 1to 4 all exhibited little increase in error rate under high temperatureand humidity, and afforded good running durability.

[0130] By contrast, Comparative Example 1, comprising colloidal silicainstead of a clay mineral treated with an organic material in thenonmagnetic support, and Comparative Example 2, in which the averagemajor axis length of the ferromagnetic metal powder contained in themagnetic layer was 120 nm, exceeding the range of the present invention,both had high error rates. In particular, the magnetic tape ofComparative Example 1 exhibited an error rate that increasedsubstantially due to changes in temperature and humidity. This wasattributed to the high variation in expansion coefficients due tochanges in temperature and humidity conditions of the nonmagneticsupport.

[0131] The flexible disks of Embodiments 5 to 8, comprising a claymineral treated with an organic material in the nonmagnetic support andcomprising ferromagnetic hexagonal ferrite powder with an average platediameter of 5 to 40 nm in the magnetic layer, all had low error rates.Further, the change in error rate due to change in temperature andhumidity conditions was small. As in Embodiments 1 to 4, this wasattributed to the low variation in expansion coefficients due totemperature and humidity conditions of the nonmagnetic support. Further,the flexible disks of Embodiments 5 to 8 also all exhibited littleincrease in error rate due to high temperature and high humidity, andafforded good running durability.

[0132] By contrast, Comparative Example 3, comprising colloidal silicainstead of a clay mineral treated with an organic material in thenonmagnetic layer, and Comparative Example 4, in which the average platediameter of the ferromagnetic hexagonal ferrite powder contained in themagnetic layer was 50 nm, exceeding the range of the present invention,both had high error rates. In particular, the flexible disk ofComparative Example 3 exhibited a high change in error rate due tochange in temperature and humidity conditions. This was attributed to alarge variation in expansion coefficients due to change in thetemperature and humidity conditions of the nonmagnetic support.

[0133] The present invention provides a magnetic recording mediumaffording low noise, a good C/N ratio, and a stable and low error rate.In particular, the particles of ferromagnetic powder in the magneticlayer in the magnetic recording medium of the present invention aresmall, permitting the achievement of high density (a high fill rate).Further, in addition to being suited to recording and reproductionsystems employing MR (magnetoresistive) heads due to the small change inthermal and moisture expansion coefficients during recording andreproduction, the magnetic recording medium of the present inventionmaintains a high C/N ratio and a low error rate even in the high-densityrecording range, and affords good running durability.

[0134] The present disclosure relates to the subject matter contained inJapanese Patent Application No. 2002-113872 filed on Apr. 16, 2002,which is expressly incorporated herein by reference in its entirety.

What is claimed is:
 1. A magnetic recording medium comprising a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder in this order on at least one surface of a nonmagnetic support, wherein said nonmagnetic support comprises a clay mineral coated with an organic material, and said ferromagnetic powder is a ferromagnetic hexagonal ferrite powder having an average plate diameter ranging from 5 to 40 nm or a ferromagnetic metal powder having an average major axis length ranging from 20 to 100 nm.
 2. The magnetic recording medium according to claim 1, wherein said ferromagnetic powder is a ferromagnetic hexagonal ferrite powder having an average plate diameter ranging from 10 to 38 nm.
 3. The magnetic recording medium according to claim 1, wherein said ferromagnetic powder is a ferromagnetic hexagonal ferrite powder having an average plate diameter ranging from 15 to 36 nm.
 4. The magnetic recording medium according to claim 1, wherein said ferromagnetic powder is a ferromagnetic metal powder having an average major axis length ranging from 30 to 90 nm.
 5. The magnetic recording medium according to claim 1, wherein said ferromagnetic powder is a ferromagnetic metal powder having an average major axis length ranging from 40 to 80 nm.
 6. The magnetic recording medium according to claim 1, wherein said clay mineral is a layered silicate compound.
 7. The magnetic recording medium according to claim 6, wherein said layer silicate compound is at least one member selected from the group consisting of smectite clay, swelling mica, and swelling vermiculite.
 8. The magnetic recording medium according to claim 6, wherein said layered silicate compound is montmorillonite, beidellite, nontronite, saponite, iron saponite, hectorite, sorconite, stibnite, bentonite, substitution products thereof, derivatives thereof, or mixtures thereof.
 9. The magnetic recording medium according to claim 6, wherein said layered silicate compound is Li-type tenorite, Na-type tenorite, Li-type tetrasilicon mica, and Na-type tetrasilicon mica, substitution products thereof, derivatives thereof, or mixtures thereof.
 10. The magnetic recording medium according to claim 1, wherein said said clay mineral coated with an organic material is a compound obtained by subjecting a layered silicate compound to the action of organic onium ions.
 11. A method of reproduction with a magnetoresistive (MR) head, wherein the reproduction is conducted on the magnetic recording medium according to claim
 1. 